1. Technical Field
This disclosure generally relates to oil and gas well drilling and the subsequent investigation of subterranean formations surrounding the well. More particularly, this disclosure relates to apparatus and methods for disengaging or “unsticking” components of a tool from the wall of the well.
2. Description of the Related Art
Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the wellbore wall.
For successful oil and gas exploration, it is necessary to have information about the subsurface formations that are penetrated by a wellbore. For example, one aspect of standard formation evaluation relates to the measurements of the formation pressure and formation permeability. These measurements are essential to predicting the production capacity and production lifetime of a subsurface formation.
One technique for measuring formation and fluid properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a wireline in electrical communication with a control system disposed on the surface. The tool is lowered into a well so that it can measure formation properties at desired depths. A typical wireline tool may include a probe that may be pressed against the wellbore wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” Using the probe, a formation tester measures the pressure of the formation fluids and generates a pressure pulse, which is used to determine the fluid mobility or the formation permeability. The formation tester tool may also withdraw a sample of the formation fluid that is either subsequently transported to the surface for analysis or analyzed downhole.
In order to use any wireline tool, whether the tool be a resistivity, porosity or formation testing tool, the drill string must be removed from the well so that the tool can be lowered into the well. This is called a “trip” uphole. Further, the wireline tools must be lowered to the zones of interest, generally at or near the bottom of the hole. The combination of removing the drill string and lowering the wireline tool downhole is time-consuming and can take up to several hours, depending on the depth of the wellbore. Because of the great expense and rig time required to “trip” the drill pipe and lower the wireline tool down the wellbore, wireline tools are generally used only when the information is absolutely needed or when the drill string is tripped for another reason, such as changing the drill bit. Examples of wireline formation testers are described, for example, in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223.
To avoid or minimize the downtime associated with tripping the drill string, another technique for measuring formation properties has been developed in which tools and devices are positioned near the drill bit in a drilling system. Thus, formation measurements are made during the drilling process and the terminology generally used in the art is “MWD” (measurement-while-drilling) and “LWD” (logging-while-drilling). A variety of downhole MWD and LWD drilling tools are commercially available.
MWD typically refers to measuring the drill bit trajectory as well as wellbore temperature and pressure, while LWD refers to measuring formation parameters or properties, such as resistivity, porosity, permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, allows the drilling company to make decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process. While LWD and MWD have different meanings to those of ordinary skill in the art, that distinction is not germane to this disclosure, and therefore this disclosure does not distinguish between the two terms. Furthermore, LWD and MWD are not necessarily performed while the drill bit is actually cutting through the formation. For example, LWD and MWD may occur during interruptions in the drilling process, such as when the drill bit is briefly stopped to take measurements, after which drilling resumes. Measurements taken during intermittent breaks in drilling are still considered to be made “while-drilling” because they do not require the drill strings to be tripped.
Formation evaluation, whether during a wireline operation or while drilling, often requires that fluid from the formation be drawn into a downhole tool for testing and/or sampling. Various sampling devices, typically referred to as probes, are extended from the downhole tool to establish fluid communication with the formation surrounding the wellbore and to draw fluid into the downhole tool. A typical probe is a circular element extended from the downhole tool and positioned against the sidewall of the wellbore. A rubber packer at the end of the probe is used to create a seal with the wellbore sidewall. Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings expand radially about the tool to isolate a portion of the wellbore therebetween. The rings form a seal with the wellbore wall and permit fluid to be drawn into the isolated portion of the wellbore and into an inlet in the downhole tool.
The tool used to evaluate the formation is susceptible to becoming stuck to the wellbore wall. The pressure of the wellbore fluid, or mud, used to form the mudcake layer must be maintained at a higher level than the pressure of the formation to prevent the formation fluid from flowing out of the formation and quickly rising to the surface. Various chemical constituents are added to the mud to increase its density and overall weight, and to increase the pressure of the wellbore fluid, referred to as the hydrostatic pressure of “mud pressure.” The difference between the mud pressure and the formation pressure is referred to as the “pressure differential.” This difference is typically 2,000 psi or less, but may reach as high as 6,000 psi. If the pressure differential is positive (the pressure is overbalanced), then the fluid and solid content of the mud will tend to flow into the formation. If the pressure differential is negative (the drawdown pressure), then the fluid and solid content of the formation will tend to flow from the inside of the formation to the wellbore and upwards toward the surface. If a positive differential is maintained, then wellbore fluid and solid particles will flow from the wellbore into the formation, and the solid particles will stack up against the wall of the wellbore. Over time, the stacked particles will create the mudcake layer that seals between the wellbore and the formation. If the mudcake layer is removed from the wall of the wellbore, and if a positive pressure differential still exists, then the contents of the wellbore again will begin to flow into the formation and a new mudcake layer will be formed. The mudcake layer may have a thickness from a fraction of millimeter to ½ inch and more, depending on the permeability of the formation, mud type, drilling operations and procedures, and the prevailing pressure differential.
If the mudcake layer is removed or disturbed while a downhole tool is transported through the wellbore, then the tool can be drawn towards the wellbore wall due to the differential pressure and become stuck to the wall. This phenomenon is known as “differential sticking.” The probability for the tool to become differentially stuck is primarily proportional to the following variables: (1) the amount of area of mudcake layer that has been removed or disturbed; (2) the amount of positive differential pressure; (3) the surface area of the tool that is in contact with the area of removed mudcake; (4) the amount of time the tool surface area is in contact with the area of removed mudcake.
In addition to the tool housing, components that are extended radially outwardly from the tool may be prone to differential sticking. During formation evaluation procedures, such as coring or formation fluid sampling, a piston and/or a probe are extended into contact with the mudcake. These extendable components may intentionally or inadvertently disrupt the seal formed by the mudcake layer, thereby exposing the component to the differential pressure. When the differential pressure is positive, it creates a force that holds the extendable component against the wellbore wall, thereby making it difficult to retract the component. Additionally, portions of the extendable component may become damaged or may break off and fall to the bottom of the wellbore, thereby interfering with subsequent drilling or other well operations. Known methods for disengaging downhole tools, such as fishing, cable pulling, and tool pushing by tubing, are overly difficult and time consuming.